1. Technical Field
The present invention relates to a variable capacitance element, a resonator, and a modulator. In particular, the present invention relates to a variable capacitance element, a resonator, and a modulator using MEMS (Micro Electro Mechanical System) technology.
2. Related Art
A variable capacitance element uses many electrical circuits such as a VFO (Variable Frequency Oscillator), a tuned amplifier, a phase shifter, and an impedance matching circuit. In recent years, variable capacitance elements are being increasingly used in portable devices, which creates a particular need for miniaturization and cost decrease. Use of variable capacitance elements at high frequency bands is also increasing. Variable capacitance elements manufactured using MEMS technology are expected to have less loss and a higher Q value than varactor diodes, which are widely used at present.
Japanese Patent Application Publication No. 2004-172504 discloses a variable capacitor manufactured using MEMS technology and having an electrostatic actuator. This variable capacitor is provided with fixed capacitor electrodes and fixed actuator electrodes formed on a substrate, and with mobile actuator electrodes and mobile capacitor electrodes supported elastically on the fixed electrodes. The mobile actuator electrodes and mobile capacitor electrodes are formed integrally, and when the mobile actuator electrodes are moved by the electrostatic force between the fixed actuator electrodes and mobile actuator electrodes, the mobile capacitor electrodes are also moved. Accordingly, the intervals between the mobile capacitor electrodes and the fixed capacitor electrodes change, so that the capacitance also changes.
Japanese Patent Application Publication No. 2004-127973 discloses a variable capacitor manufactured using MEMS technology and having actuators that use piezoelectric materials. This variable capacitor is provided with a pair of mobile electrodes that both have actuators, and the capacitance is changed by bringing these electrodes close together or moving them further apart to change the space therebetween.
The above variable capacitor disclosed in Japanese Patent Application Publication No. 2004-172504 uses electrostatic actuators. In order for the electrostatic actuators to operate effectively, the actuator electrodes must have sufficient surface area. Therefore, the overall area of the variable capacitor including these electrodes is large. Furthermore, the electrostatic actuator has a drive force that changes greatly depending on the distance between the electrodes, and so it is difficult for the mobile electrodes to find a stroke. Yet further, a relatively high voltage around 10 V is desired for driving the electrostatic actuator, and so it is difficult to use this actuator in a circuit having a battery as a power source.
The above variable capacitor disclosed in Japanese Patent Application Publication No. 2004-127973 has piezoelectric actuators, and can therefore be driven by a relatively low voltage. This variable capacitor also has a faster response than the capacitor having electrostatic actuators. However, in this variable capacitor, the dimensions of the mobile electrodes increase relative to the size of the capacitance. Therefore, the mass of the mobile electrodes, which are being moved by the actuator, increases, thereby slowing down the operation. The only way to maintain the same response speed is to increase the size of the actuators.